Stable carbon and nitrogen isotopes were sampled in representative environments of southern and eastern Gulf of Mexico to trace carbon and nitrogen sources and processes affecting them. Sampled sites include a hydrocarbon seep area, a coastal zone influenced by terrestrial discharge, a productive oil field, a coral reef, and a deepwater environment. In Cantarell oil field, δ13C and δ15N values of suspended particulate matters, sediments, and benthic organisms show that the principal carbon source to the benthic food web is the downward flux of upper-layer primary production. In the coastal zone, the isotopic terrestrial signature of suspended particles across the low salinity plume indicates that the terrestrial contribution in nearshore waters is progressively diluted by marine organic matter. Hydrocarbon concentrations and δ13C values from a Bay of Campeche hydrocarbon seep sediment core suggest that the seep contributes to about 72.4% petrogenic carbon to its surface sediment layer. The δ13C values in corals suggest a carbon source from fixation by zooxanthellae. In the eastern Gulf, organic carbon (Corg) and total nitrogen (TN) concentrations and isotopes are indicative of low terrestrial contribution, and the principal long-term nitrogen source to primary producers appears to be nitrate diffusing from the thermocline into the photic zone.
Part of the book: Advances in the Studies of the Benthic Zone
Stable nitrogen and carbon isotopes were used to trace the interaction between atmospheric particles < 2.5 mm in diameter (PM2.5) with atmospheric physical variables and atmospheric chemical species in an urban environment. PM2.5 were collected daily at two sites in Mexico City during three dry seasons during two-week periods. PM2.5 varied between 10 and 70 μg/m3, with the highest concentrations occurring during low-speed southerly winds and the lowest during high-speed easterly winds. Stable carbon isotope composition (δ13C) showed that the main carbon source of PM2.5 included emissions from fossil fuel combustion, along with low-molecular-weight carbon emissions and suspended dust. Stable nitrogen isotope values (δ15N) in PM varied between −9.9 and 21.6‰. The most 15N-enriched particles generally occurred during low wind speeds and correlated significantly with hourly averaged ambient NOx and NO2 concentrations. Simultaneous samples from MER (commercial site) and XAL (industrial site) showed that PM2.5 mass concentration was generally lighter and the carbon and nitrogen isotopic compositions were heavier at the commercial site relative to the industrial site. The δ15N of PM2.5 increased with the %N in PM2.5concordant with an isotopic fractionation during gas-to-particle condensation. Results suggest that wind speed, along with the nitrogen emission source, determines the nitrogen isotopic composition of PM2.5.
Part of the book: Air Pollution